tools/libs/light: Add vlan field to libxl_device_nic

tools/libs/light: Add vlan field to libxl_device_nic

Add `vlan' string field to libxl_device_nic, to allow a VLAN
configuration to be specified for the VIF when adding it to the
bridge device.

Update libxl_nic.c to read and write the vlan field from the
xenstore.

This provides the capability for supported operating systems (e.g.
Linux) to perform VLAN filtering on bridge ports.  The Xen
hotplug scripts need to be updated to read this information from
the xenstore and perform the required configuration.

Signed-off-by: Leigh Brown <leigh@solinno.co.uk>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>